One important mechanism of transcriptional control is genomic imprinting, a process by which a gene is expressed from only one parental allele. Loss of imprinting is implicated in human disease and cancer. Alterations in regulation of the imprinted genes IGF2 and HI9 have been shown to contribute to Beckwith-Wiedemann Syndrome, a syndrome of overgrowth and tumor development. These genes are expressed from different parental alleles, but their imprinted expression is regulated by common DNA elements. An important molecular difference between alleles is differential methylation of CpGs in the H19 5' region. This differentially methylated domain (DMD) is required for imprinted expression of both HI9 and Igf2 and methylation of the DMD is correlated with the silencing of HI9. How this DNA methylation is interpreted and translated into a repressed state is unknown. A family of proteins, the Methyl-CpG binding proteins (Mbd), has been identified that may be the link between methylation and silencing. I propose to test the hypothesis that Mbd proteins are required for the silencing of imprinted genes. I will employ an RNAi approach to reduce Mbd activity in early embryos, and then determine the effect reduction of Mbd activity has on imprinted gene expression and methylation. I will also identify which Mbds are bound to repressive methylated chromatin at imprinted loci in embryonic cell lines.